The present invention relates to a point feeder for feeding additives such as aluminium oxide or fluoride to aluminium electrolysis cells, particularly electrolysis cells of the Soederberg type, comprising an anode with a basically rectangular casing of steel or cast iron. The additives are fed to the electrolytic bath of the cell through a hole in the bath crust, which hole is made by means of a crust breaker, the crust beaker being a crow bar connected to a piston/cylinder device.
When producing aluminium according to the Hall-Heroult process one can distinguish between two different design principles. Thus, the electrolysis cells can be provided with self-baking anodes, the socalled Soederberg anodes, which are continuous, or with readily baked anodes of carbon, socalled prebaked anodes, which have to be exchanged during the electrolysis process as they are gradually consumed.
For both types of cells the anode is disposed above a cathode which principly has the same constructional design and comprises a rectangular steel shell which on the inside is provided with a thermal insulation of refractory bricks on the bottom and sides. On the high temperature side, on the inside of the thermal insulation, there is provided a carbon lining. This is designed as a shallow container containing an electrolytic bath and precipitated aluminium. In the carbon lining are disposed steel bars, so-called cathode bars, which provide the electric connection between the carbon cathode and outer busbars.
The electrolytic bath comprises aluminum oxide dissolved in melted cryolite with small amounts of additives, basically aluminum fluoride and calcium fluoride. During the electrolysis process, the aluminum oxide is used as it is decomposed to oxygen, which immediately reacts with the carbon of the anode, and aluminium, which due to the gravitational force sinks to the bottom of the cathode. The other additives are to some extent also used, and to maintain the chemical balance in the electrolytic bath, new additives have to be supplied to replace the ones being consumed.
It has for a long time been commonly known to feed additives to electrolytic cells by means of one or more point feeders being provided on the anode (through the carbon of the anode), on the side of the anode, or if it is a prebake cell, between the anode carbons. Such arrangements are, for instance, shown in Norwegian patent specification No. 844448, which is from 1954.
The point feeders have not found any broad application until the last decade, and their application has to a large extent been limited to the prebake cells. The reason for not using point feeders at an earlier stage seems to reside in the fact that there has not been any well developed control systems, and that the feeders per se have not been sturdy enough to resist the harmful environment in the electrolysis cells.
The advantages with the point feeders are several. The cells can be more optimally operated by keeping the chemical balance of the electrolytic bath at a uniform level. The manual work and the maintenance of the cells are further reduced and the gas and dust demission is, to a large extent, eliminated as the cells are completely closed.
As to the Soederberg cells, the feeding of these is still accomplished by means of the conventional method, by cutting the crust between the anode and the side of the cell by means of a crust breaker, whereafter the aluminium oxide is supplied to the melt from a vehicle or the like.
The point feeders being developed for the cells, with the prebaked anodes, cannot immediately be used for the Soederberg cells as the anode designed for the two cell types are different. As opposed to the cells with the prebaked anodes where the point feeders can be disposed between the anode carbons and where the whole anode construcation is built in under a cover, the Soederberg cells are completely open to their surroundings, and the only real possibility of localizing the point feeders would be to place them between the anode and the side crust of the cell. One of the main reasons for not having used point feeding on the Soederberg cells before are that there has not been developed point feeders which can be used in this area.
In Norwegian patent application No. 874538 is shown a method and a device for point feeding Soederberg cells where the point feeder is provided on an incision on the anode casing. If this solution is to be used on existing Soederberg cells, the anode casing has to be redesigned, which will involve large expenses. Besides, the point feeder is of the conventional type where a crust breaker comprising a piston/cylinder device is provided within a vertical pipe which is open downwardly towards the electrolytic bath, and where the additives are supplied through a pipe stub via the vertical pipe. The piston rod for the piston/cylinder device is directly exposed to gas, dust and heat from the electrolytic bath, and this, together with the side forces which the crow bar is exposed to, will soon lead to leakages in the packing between the piston rod and the cylinder due to wear.